THE EFFECTS OF CELLS ON OXYGEN-TRANSFER IN BIOREACTORS - PHYSICAL PRESENCE OF CELLS AS SOLID PARTICLES

Oxygen transfer into broths of non-respiring live baker's yeast was studied at different cell concentrations, agitation speeds and aeration rates. Under all the studied conditions, the physical presence of cells as solid particles was found to reduce the oxygen transfer rate. The effect of physical blockage by the less permeable cells accumulated near the bubble surface was dominant in the studied systems, while the hydrodynamic enhancement by the ''snowball'' effect (G.F. Andrews, J.P. Fonta, E. Marrotta and P. Stroeve, Chem. Eng. J., 29 (1980) B39-B46, B47-B55) was insignificant. The reduction in volumetric oxygen transfer coefficient k(L)a increased with cell concentration, but with decreasing marginal effect. Higher agitation speeds and the lower aeration rate used in the study (1 vs. 2 vvm), which produced smaller bubbles with higher cell capture efficiencies, were also found to promote the blocking effect of non-respiring cells.